Conventional vehicle lighting units have employed bulbs as a light source and examples of such bulbs may include HB3 type, HB4 type, HIR2 type, etc. Such a bulb can be detachably fixed to the main body of a vehicle lighting unit, such as to a reflector, a housing, or the like with a bayonet mechanism. Examples of this type of vehicle lighting unit may include those described in Japanese Patent Application Laid-Open No. Hei. 10-031901 and Japanese Patent No. 2978795.
A description will now be given of one example of a fixation structure for fixing a bulb by means of a bayonet scheme (bayonet mechanism) with reference to FIG. 1.
FIG. 1 is an exploded perspective view illustrating an example of a conventional bulb fixation structure of a vehicle lighting unit. In the illustrated example, the vehicle lighting unit has a main body, which is a resin-made reflector 1. The reflector 1 has an opening serving as a bulb attachment hole 2 in a circle shape on its rear surface. The vehicle lighting unit has a cylindrical holder 3 that is integrally formed with and projected around the peripheral edge of the bulb attachment hole 2 on the rear surface of the reflector 1. The holder 3 has an opening end face 3c, to which a ring-shaped supporter spring 4 formed of a thin metal plate, such as SUS with a thickness of 0.6 mm, is attached with two screws 5. The supporter spring 4 has an engagement claw 4b at one location of the inner periphery thereof by cutting and rising part of the supporter spring 4. Further, the holder 3 of the reflector 1 has an engagement groove 3b formed at one location of the inner periphery thereof. The engagement claw 4b can engage with the engagement groove 3b. 
Furthermore, the supporter spring 4 has three cut-out arc-shaped claw escape holes 4A at three locations of the inner periphery thereof in the circumferential direction. The supporter spring 4 further has three engagement springs 4c adjacent to the respective claw escape holes 4A in the circumferential direction by cutting and bending a part of the plate-shaped supporter spring 4 toward the holder 3 as illustrated in FIG. 1.
On the other hand, a disc-shaped flange 8 is provided to the outer periphery of the base portion of the bulb 6 (where a socket 7 is connected) so as to be perpendicularly erected therefrom. The flange 8 has three arc-shaped engagement claws 8A at three locations of the periphery thereof so as to project radially outward. The shape of the engagement claw 8A coincides with the claw escape hole 4A of the supporter ring 4.
In the conventional bulb fixation structure of the vehicle lighting unit, when the bulb 6 is fixed to the reflector 1, first, the tip end of the bulb 6 is inserted into the bulb attachment hole 2 of the reflector 1 and the three engagement claws 8A of the flange 8 of the bulb 6 are allowed to be matched to the three claw escape holes 4A of the supporter spring 4 and pass therethrough, thereby incorporating these three engagement claws 8A inside the holder 3 of the reflector 1. Then, the flange 8 of the bulb 6 is urged against an opening end face 3d of the bulb attachment hole 2 opened within the holder 3 of the reflector 1. After that, the bulb 6 in this state is rotated by a predetermined angle in a clockwise direction in FIG. 1 around its axial center. By doing so, the three engagement claws 8A of the flange 8 are engaged with the respective engagement springs 4c of the supporter spring 4. In this manner, the bulb 6 is fixed to the reflector 1 by means of the bayonet mechanism. Specifically, this is because the flange 8 of the bulb 6 is interposed in between and held by the opening end face 3d of the bulb attachment hole 2 and the engagement springs 4c of the supporter spring 4.
In the conventional bulb fixation structure as illustrated in FIG. 1, during the fixation operation in which the three engagement claws 8A are allowed to be matched to the three claw escape holes of the supporter spring 4 and pass therethrough and then the bulb 6 is rotated by a predetermined angle around its axial center, since the supporter spring 4 is made of a thin metal plate such as a SUS with a thickness of 0.6 mm and is attached by screws 5 only at two locations, the following problems arise. Incidentally, when the supporter spring 4 is fixed with the use of normal screws 5, the supporter spring 4 should be fixed at two locations with two screws 5 otherwise the screws 5 and the bulb 6 interfere with each other.
The problem is such that the supporter spring 4 tends to float when it is fixed only by two screws 5. In this situation, the three engagement claws 8A cannot pass through the three claw escape holes 4A of the supporter spring 4 simultaneously. In this case, for example, part of (one of, for example) the engagement claws 8A may stride over the claw escape hole 4A of the supporter spring 4 and ride on the supporter spring 4 itself. This results in insufficient fixation of the bulb 6 to the reflector 1 due to wrong assembling.